The present invention relates to a process of treating organic wastes for recovering methane from organic wastes by fermentation effect of anaerobic bacteria.
In the past, an anaerobic digestion process (methane fermentation process) has been employed for treating such organic wastes as sewage sludge, human and livestock wastes and food processing drainage. This process not only suits for treating wastes which contain much water and which present some difficulties in incineration, but also has a feature facilitating recovery of methane, a source of clean energy. In western countries, gas power generation has been practiced since long, and in Japan as well it has become popular in recent years. As this anaerobic digestion process, however, depends on the principle of anaerobic fermentation by anaerobic bacteria having a lesser speed of fermentation than that of aerobic bacteria which are used for aerobic treatment, it takes more than a week even in a quickest treatment, or usually requires such a long period of time for treatment (time of residence in fermentation tank) as 15 to 50 days. On account of growing restriction in recent years on the sites of new treatment installations, it now requires a speedier treatment and a more compact facility.
It is known that in anaerobic digestion the organic matters are only decomposed into methane when they come through two different fermentations; i.e. first, a liquefying fermentation wherein the organic matters are turned into such volatile fatty acids of low molecular weight as acetic acid, propionic acid and butyric acid by the effect of facultative anaerobic bacteria (liquefying bacteria or septic bacteria), and second, a gasifying fermentation wherein thus generated fatty acids are converted into methane through the effect of obligatory anaerobic bacteria (gasifying bacteria or methane bacteria). The digestion process presently being practiced is a mixed fermentation method which carries out two fermentations, namely, liquefaction and gasification within the fermentation tank under the co-presence of these two groups of bacteria. Recently, a study is being made for a separate fermentation method which performs these two fermentation independently from each other, as a method for making the fermentation time further shorter than that of the mixed fermentation method.
In general, as one of the means to improve fermentation speed in a continuous waste treatment, there is one means wherein the concentration of bacteria in the fermentation tank is increased by recycling the bacteria back to the fermentation tank after recovering them. This means is very commonly used in the activated sludge process. In the activated sludge process, first, the waste water having all solid matters removed by a sedimentation process is treated. In the process of aerobic treatment, the aerobic bacteria which proliferate in the aeration tank flocculate into flocs that precipitate by gravitation and can very easily be separated from the waste water under treatment. In this case, most of the excess sludge is of aerobic bacteria, so that to recycle it after recovery to the aeration tank is very useful from the standpoint of improving the treatment efficiency.
As for the anaerobic digestion process as well, there is a case, wherein the solid fraction produced from fermentation slurry through solid-liquor separation by a sedimentation process, i.e. digested sludge, is recycled back to the fermentation tank. In the anaerobic digestion, however, since its purpose is to decompose solids unlike the activated sludge process, feedstock is charged into the fermentation tank without removing any solid matters contained therein. Further, in case of anaerobic digestion, most of the solids within the digestion slurry are non-digestible solids as the yield of bacteria is less than 1/10 of aerobic bacteria. Recycling of digested sludge, therefore, invites an increased concentration of solids in the fermentation tank and has a drawback in increasing the dimension of the tank and giving added burden on the stirrer. The unfavorable effect of this drawback is extremely explicit in case of the feedstock involving much non-digestable solids such as city garbage which is expected to become a practicable feedstock for this process, let alone those sewage sludge and human and livestock wastes to which the process is presently applied.
Incidentally, recycling the digested sludge back to the fermentation tank is being praticed only because, in the first place, both of the anaerobic bacteria have been presumed to have been deposited on the undecomposed solids in the digested sludge and have been concentrated, and, secondly, even if the bacteria in the waste water under treatment are supposed to exist in a suspended situation, no practicable means for concentration-separation without biologically deactivating the bacteria has not so far been identified. It has not been clarified yet whether a greater proportion of those two bacteria is dispersed in the solid fraction of slurry (digested sludge) or whether in the liquor fraction (treated water), due to a lack of any reliable analysis method.
Thereupon, the present inventors had checked by microscope in what manner the bacteria exist in various kinds of anaerobic digested slurry, and, as a result, they found that those anaerobic bacteria suspended in the liquor are far greater in their proportion than those which are deposited on such solids which are not easily decomposable as cellulosic fibers.
Based on the aforementioned new knowledge, and as a result of their serious efforts for identifying a process highly practicable for recovering cells of both anaerobic bacterial cells contained in the anaerobically digested liquor, they acquired another new knowledge as cited here below. When excess activated sludge which is produced by treating organic waste water with activated sludge was added to the digestion treatment water and stirred/mixed under an open atmosphere, it was discovered that (1) both anaerobic bacteria were effectively deposited on excess activated sludge, and (2) they could easily be precipitated/separated together with excess activated sludge through gravity sedimentation. Upon testing the fermentation activity of both anaerobic bacteria thus recovered, it was made known that the beginning of fermentation delayed three to six days and the fermentation activity as well was low. In other words, it was noted that even if physically the bacterial cells could be recovered, their biological activity was damaged. Then through further research efforts, it was finally clarified that if the stirring be performed under an anaerobic conditions, there would be no delay in the beginning of fermentation and the fermentation activity could be high enough.
The present invention has been the product based on a number of new discoveries as mentioned above.